The present invention relates to devices and methods for calibrating the pulse duration of a signal source, i.e. for calibrating the temporal wave form of an output signal of a signal source. In particular the present invention relates to such devices and methods that may be used in an on-chip calibration and allow, for example, measuring and adjusting a duty cycle of the output signal of a signal source.
In order to calibrate the temporal wave form of an output signal of an integrated circuit, an external measurement/test system has been used in the prior art. In particular with wave forms having, in temporal terms, short signal cycles, calibration of such a wave form requires an external measurement/test system with high accuracy. Such a measurement/test system, however, is costly, it requires space, and a very high process-related expense is required for connecting the integrated signal sources to the measurement/test system.
It is therefore the object of the present invention to provide a device and a method by which signal calibration of a signal source may be carried out at a minimum process and device-related expense.
The present invention provides a device for calibrating the pulse duration of the output signal of a signal source, comprising:
voltage comparison means having a first input, a second input and an output;
means for providing a reference voltage connected to the first input of the voltage comparison means;
charge storing means, the charge state of which is adjustable as a function of the pulse duration of a pulse of the signal source, the charge storing means being connected to the second input of the voltage comparison means; and
processing means for adjusting the pulse duration as a function of the reference signal output at the output of the voltage comparison means.
The present invention further provides a method of calibrating the pulse duration of an output signal of a signal source, comprising:
providing a reference voltage having a pre-determined level;
adjusting the charge state of a charge storing means as a function of the pulse duration of a pulse from the signal source;
comparing the reference voltage with a voltage dependent on the charge state of the charge storing means; and
adjusting the pulse duration as a function of the comparison of the reference voltage with the voltage dependent on the charge state of the charge storing means.
The present invention thus makes it possible to convert a time period, i.e. the pulse duration of the pulse output from the signal source, by means of the charge storing means, which is designed, for example, as a capacitor, into a voltage value so as to subsequently compare this voltage value with a pre-determined reference voltage. The result of the comparison will then allow to draw conclusions as to whether the pulse duration of the pulse being set is too low or too high and must therefore be set to a higher or a lower value. By converting a time period into a voltage value, the inventive method allows the use of simple electronic components and renders the use of expensive, complicated time measurement devices superfluous.
While in the simplest case the charge state of the charge storing means is set or changed only as a function of the pulse duration of a pulse, it is also possible in accordance with an advantageous embodiment of the invention to set the charge state of the charge storing means as a function of a signal cycle consisting of the pulse and the pulse intermediary period intervening before the occurrence of a next pulse. In such a case, for example, charges are applied to the charge storing means during a pulse of the signal cycle, while charges are taken from the charge storing means during the pulse intermediary period. If there is a desired duty cycle, i.e., a ratio of pulse duration and pulse intermediary period, of, for example, 1:1, there is thus a net charge change during a signal cycle of zero. In such a case the pulse duration need not be set. However, if net charging or net discharging of the charge storing means arises due to a deviation from a desired duty ratio, a corresponding counteracting setting of the pulse duration of the signal source becomes necessary, which represents a calibration of same.
In accordance with the invention, a plurality of signal cycles of the output signal of the signal source are passed before a comparison with the reference voltage is carried out and before the pulse duration is set, so as to effect a summing up of the net charge change effected in each signal cycle and to compare a voltage dependent on these summed-up net charge changes with the reference voltage. This enables the reliable detection of even very small deviations from a desired duty cycle. In preparation the charge storing means is preferably biased to a pre-determined charge state.
In the process, the comparison of the voltage based on the summed-up net charge change with the reference voltage can be effected after a pre-determined number of cycles or after a pre-determined time period. The number of cycles or the time period used is selected to be sufficiently large to ensure that the system will have reached a steady state thereafter.
As a function of the result of the comparison carried out after one or a plurality of signal cycles, the pulse duration of the pulse of the signal source is either decreased or increased so as to achieve an approximation to the desired pulse duration and/or a desired duty cycle. This procedure is preferably repeated several times so as to obtain an iterative approximation to the desired result. To this end, the control quantity by which the pulse duration is increased or decreased is reduced, for example halved, with each repetition. Doing this, the method is repeated as many times as are needed to achieve the desired value, i.e. the target value, with the desired accuracy.
In accordance with advantageous embodiments of the present device, the comparison means has a comparator, and the charge storing means has a capacitor.
In accordance with a further advantageous embodiment of the invention the charge storing means has a capacitor, one electrode of which is connected to a biasing means, and the other electrode of which is connected to the output of the signal source via a resistor. The connection point between the capacitor and the resistor is connected to the second input of the voltage comparison means.
The charge state of the charge storing means may be set using a constant current source instead of the above-mentioned RC member for temporal integration of a voltage. In this case the output signal of the signal source is compared to a further reference voltage, wherein, when the output signal exceeds a pre-determined level, i.e. during the pulse duration, loading of the charge storing means by the constant current source is effected. If the output signal falls short of the further reference voltage, i.e. during a pulse intermediary duration, the charge storing means is discharged via the constant current source.
The use of the controlled constant current source allows charging or discharging of the capacitor with a linear characteristic, in contrast with the characteristic in the form of an exponential function (e-function), when the capacitor is charged or discharged directly via a resistor through the pulse of the signal source to be calibrated.
In accordance with further advantageous embodiments of the invention an attenuator in the form of a low-pass is connected between the charge storing means and the voltage comparison means, which attenuator has a resistor and a capacitor, for example.
The respective reference voltages may be provided directly by a connection to corresponding voltage sources of the inventive device. In accordance with the invention, however, preferably corresponding voltage divider circuits are used which allow to obtain the required voltages from the voltages existing on an chip in the usual manner. Thus the inventive calibration device may be integrated in an on-chip manner without any substantial device-related expense.
In accordance with the invention no external measurement/test system is thus required for calibrating the pulse duration of a signal source. In accordance with the invention the cost as well as the space requirement that would be associated with such an external measurement/test system may be saved. After all the present invention enables constant re-calibration of signal sources arranged on a chip if the calibration means is integrated on the chip, which is readily possible due to the structure of same in terms of circuitry.